The reality that we live in exists in the same dimensional space as another reality inhabited by demons. We cannot see them, and yet they are there watching us. Where we sleep, eat, work, where are children play, in the bathroom when we take a crap, etc. They inhabit every crevice of our world, separated by a thin barrier between realities that protect us from them.
These demons are born from strong human emotions. Hate, fear, anger, etc, these intense drives that we feel resonate with the other reality, compounding together to form these entities. Most of these demons evaporate soon after they are formed. The strongest of these feelings create the strongest demons, which continue to exist and feed off of human emotions that created them.
A cult devoted to an ancient entity, Dagon, has conducted a millenia-long plan to allow the demons into our reality. Through selective breeding program enacted over thousands of years, a child would be born of a virgin woman that would act as a vessel for the demonic god. This "mother of chaos" heads this sinister cult and has kept herself pure for this very purpose. This dark messiah would be born with Dagon's soul in the form of a human, and act as a Trojan horse who would eventually break the barrier separating realities.
However, there is a problem. This child has no memories of its demonic existence, and considers itself fully human. The cult, in an attempt to bring the demon forth, subjected the child to various kinds of trauma. After walking away from the cult, it sought to live a normal life. As time passed, Dagon began to emerge as a second personality in the subconscious. The story revolves around the human and demon personalities of this individual fighting and trying to supress each other for domination, with one wanting to fulfill it's destiny and the other trying to stay human.
When Christ was born, he had no problem remembering what he was or what his purpose is. In some stories, he even boasted about it. This case is different. How would reincarnation lead to the formation of two personalities? A good answer would give a biological reason as to why this second personality would come to the forefront after remaining supressed early in life.